This artist's illustration shows the exoplanet SPECULOOS-3 b orbiting its red dwarf star. The planet is as big around as Earth, while its star is slightly bigger than Jupiter – but much more massive. The planet is a prime candidate for follow-up studies with the JWST.
Credit: NASA/JPL-Caltech
Red dwarf stars, also known as M-dwarfs, dominate the Milky Way’s stellar population. They can last for 100 billion years or longer. Since these long-lived stars make up the bulk of the stars in our galaxy, it stands to reason that they host the most planets.
Astronomers examined one red dwarf star named SPECULOOS-3, a Jupiter-sized star about 55 light-years away, and found an Earth-sized exoplanet orbiting it. It’s an excellent candidate for further study with the James Webb Space Telescope.
Artist's view of old white dwarfs surrounded by planetary debris. Credit: University of Warwick/Dr Mark Garlick
Astronomers have found plenty of white dwarf stars surrounded by debris disks. Those disks are the remains of planets destroyed by the star as it evolved. But they’ve found one intact Jupiter-mass planet orbiting a white dwarf.
Are there more white dwarf planets? Can terrestrial, Earth-like planets exist around white dwarfs?
This artist's illustration of Kepler 22-b, an Earth-like planet in the habitable zone of a Sun-like star about 640 light years (166 parsecs) away. Credit: NASA/Ames/JPL-Caltech
Despite the fact that we’ve discovered thousands of them, exoplanets are hard to find. And some types are harder to find than others. Naturally, some of the hardest ones to find are the ones we most want to find. What can we do?
Keep working on it, and that’s what a trio of Chinese scientists are doing.
An artist's illustration of a rogue planet, dark and mysterious. Image Credit: NASA
Scientists have found what appear to be rogue planets hidden in old survey data. Their results are starting to define the poorly-understood rogue planet population. In the near future, the Nancy Grace Roman Space Telescope will conduct a search for more free-floating planets, and the team of researchers developed some methods that will aid that search.
Between 2021 and 2024, the James Webb(JWST) and Nancy Grace Roman (RST) space telescopes will be launched to space. As the successors to multiple observatories (like Hubble, Kepler, Spitzer, and others), these missions will carry out some of the most ambitious astronomical surveys ever mounted. This will range from the discovery and characterization of extrasolar planets to investigating the mysteries of Dark Matter and Dark Energy.
In addition to advanced imaging capabilities and high sensitivity, both instruments also carry coronagraphs – instruments that suppress obscuring starlight so exoplanets can be detected and observed directly. According to a selection of papers recently published by the Journal of Astronomical Telescopes, Instruments, and Systems (JATIS), we’re going to need more of these instruments if we truly want to really study exoplanets in detail.
Earth as seen by the JUNO spacecraft in 2013. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill.
To date, astronomers have confirmed the existence of 4,422 extrasolar planets in 3,280 star systems, with an additional 7,445 candidates awaiting confirmation. Of these, only a small fraction (165) have been terrestrial (aka. rocky) in nature and comparable in size to Earth – i.e., not “Super-Earths.” And even less have been found that are orbiting within their parent star’s circumsolar habitable zone (HZ).
In the coming years, this is likely to change when next-generation instruments (like James Webb) are able to observe smaller planets that orbit closer to their stars (which is where Earth-like planets are more likely to reside). However, according to a new study by researchers from the University of Napoli and the Italian National Institute of Astrophysics (INAF), Earth-like biospheres may be very rare for exoplanets.
Most exoplanets orbit red dwarf stars because they're the most plentiful stars. This is an artist's illustration of what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credits: NASA/JPL-Caltech
In recent decades, over 4,000 extrasolar planets have been confirmed beyond our Solar System. With so many planets available for study, astronomers have learned a great deal about the types of planets that exist out there and what kind of conditions are prevalent. For instance, they have been able to get a better idea of just how common habitable planets are (at least by our standards).
As it turns out, a surprisingly high number of planets out there could support life. That is the conclusion reached by a team of astronomers and planetary scientists who conducted a study of the possible sizes of habitable zones (HZ) based on stellar classification. After considering many planets could stably orbit within them, they came to the conclusion that stars with no Jupiter-sized gas giants can have as many as seven habitable planets!
A new study led by researchers from OU indicates that the outer planets could be why Mars is significantly smaller than Earth. Credit: NASA
For centuries, astronomers and scientists have sought to understand how our Solar System came to be. Since that time, two theories have become commonly-accepted that explain how it formed and evolved over time. These are the Nebular Hypothesis and the Nice Model, respectively. Whereas the former contends that the Sun and planets formed from a large cloud of dust and gas, the latter maintains the giant planets have migrated since their formation.
This is what has led to the Solar System as we know it today. However, an enduring mystery about these theories is how Mars came to be the way it is. Why, for example, is it significantly smaller than Earth and inhospitable to life as we know it when all indications show that it should be comparable in size? According to a new study by an international team of scientists, the migration of the giant planets could have been what made the difference.
For over a decade, astronomers have been operating under the assumption that shortly after the formation of the Solar System, the gas and ice giants of the outer Solar System (Jupiter, Saturn, Uranus and Neptune) began to migrate outward. This is the substance of the Nice Model, which asserts that this migration had a profound effect on the evolution of the Solar System and the formation of the terrestrial planets.
This model – named for the location of the Observatoire de la Côte d’Azur (in Nice, France), where it was initially developed – began as an evolutionary model that helped explain the observed distributions of small objects like comets and asteroids. As Matt Clement, a graduate student in the HL Dodge Department of Physics and Astronomy at the University of Oklahoma and the lead author on the paper, explained to Universe Today via email:
“In the model, the giant planets (Jupiter, Saturn, Uranus and Neptune) originally formed much closer to the Sun. In order to reach their current orbital locations, the entire solar system undergoes a period of orbital instability. During this unstable period, the size and the shape of the giant planet’s orbits change rapidly.”
For the sake of their study, which was recently published in the scientific journal Icarus under the title “Mars Growth Stunted by an Early Giant Planet Instability“, the team expanded on the Nice Model. Through a series of dynamical simulations, they attempted to show how, during the early Solar System, the growth of Mars was halted thanks to the orbital instabilities of the giant planets.
The purpose of their study was also to address a flaw in the Nice Model, which is how the terrestrial planets could have survived a serious shake up of the Solar System. In the original version of the Nice Model, the instability of the giant planets occurred a few hundred million years after the planets formed, which coincided with the Late Heavy Bombardment – when the inner Solar System was bombarded by a disproportionately large number of asteroids.
This period is evidenced by spike in the Moon’s cratering record, which was inferred from an abundance of samples from the Apollo missions with similar geological dates. As Clement explained:
“A problem with this is that it is difficult for the terrestrial planets (Mercury, Venus, Earth and Mars) to survive the violent instability without being ejected out of the solar system or colliding with one another. Now that we have better, high resolution images of lunar craters and more accurate methods for dating the Apollo samples, the evidence for a spike in lunar cratering rates is diminishing. Our study investigated whether moving the instability earlier, while the inner terrestrial planets were still forming, could help them survive the instability, and also explain why Mars is so small relative to the Earth.”
Clement was joined by Nathan A. Kaib, a OU astrophysics professor, as well as Sean N. Raymond of the University of Bordeaux and Kevin J. Walsh from the Southwest Research Institute. Together, they used the computing resources of the OU Supercomputing Center for Education and Research (OSCER) and the Blue Waters supercomputing project to perform 800 dynamical simulations of the Nice model to determine how it would impact Mars.
These simulations incorporated recent geological evidence from Mars and Earth that indicate that Mars’ formation period was about 1/10th that of Earth’s. This has led to the theory that Mars was left behind as a “stranded planetary embryo” during the formation of the Sun’s inner planets. As Prof. Kaib explained to Universe Today via email, this study was therefore intended to test how Mars emerged from planetary formation as a planetary embryo:
“We simulated the “giant impact phase” of terrestrial planet formation (the final stage of the formation process). At the beginning of this phase, the inner Solar System (0.5-4 AU) consists of a disk of about 100 moon-to-mars-sized planetary embryos embedded in a sea of much smaller, more numerous rocky planetesimals. Over the course of 100-200 million years the bodies making up this system collide and merge into a handful (typically 2-5) rocky planetary mass bodies. Normally, these types of simple initial conditions build planets on Mars-like orbits that are about 10x more massive than Mars. However, when the terrestrial planet formation process is interrupted by the Nice model instability, many of the planet building blocks near the Mars region are lost or tossed into the Sun. This limits the growth of Mars-like planets and produces a closer match to our actual inner solar system.”
Size comparison between Earth and Mars. Credit: NASA
What they found was that this revised timeline explained the disparity between Mars and Earth. In short, Mars and Earth vary considerably in size, mass and density because the giant planets became unstable very early in the Solar System’s history. In the end, this is what allowed Earth to become the only life-bearing terrestrial planet in the Solar System, and for Mars to become the cold, desiccated and thinly-atmosphered place that it is today.
As Prof. Kaib explained, this is not the only model for explaining the disparity between Earth and Mars, but the evidence all fits:
“Without this instability, Mars likely would have had a mass closer to Earth’s and would be a very different, perhaps more Earth-like, planet compared to what it is today,” he said. “I should also say that this is not the only mechanism capable of explaining the low mass of Mars. However, we already know that the Nice model does an excellent job of reproducing many features of the outer Solar System, and if it occurs at the right time in the Solar System’s history it also ends up explaining our inner Solar System.”
This study could also have drastic implications when it comes to the study of extra-solar systems. At present, our models for how planets form and evolve are based on what we have been able to learn from our own Solar System. Hence, by learning more about how gas giants and terrestrial planets grew and assumed their current orbits, scientists will be able to create more comprehensive models of how life-bearing planets could merge around other stars.
It certainly would help narrow the search for “Earth-like” planets and (dare we dream?) planets that support life.
Artist's concept of a collision between proto-Earth and Theia, believed to happened 4.5 billion years ago. Credit: NASA
For decades, scientists have pondered how Earth acquired its only satellite, the Moon. Whereas some have argued that it formed from material lost by Earth due to centrifugal force, or was captured by Earth’s gravity, the most widely accepted theory is that the Moon formed roughly 4.5 billion years ago when a Mars-sized object (named Theia) collided with a proto-Earth (aka. the Giant Impact Hypothesis).
However, since the proto-Earth experienced many giant-impacts, several moons are expected to have formed in orbit around it over time. The question thus arises, what happened to these moons? Raising this very question, a team an international team of scientist conducted a study in which they suggest that these “moonlets” could have eventually crashed back into Earth, leaving only the one we see today.
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
Red dwarf stars have become a major focal point for exoplanet studies lately, and for good reason. For starters, M-type (red dwarf) stars are the most common type in our Universe, accounting for 75% of stars in the Milky Way alone. In addition, in the past decade, numerous terrestrial (i.e rocky) exoplanets have been discovered orbiting red dwarf stars, and within their circumstellar habitable zones (“Goldilocks Zones”) to boot.
This has naturally prompted several studies to determine whether or not rocky planets can retain their atmospheres. The latest study comes from NASA, using data obtained by the Mars Atmosphere and Volatile Evolution (MAVEN) orbiter. Having studied Mars’ atmosphere for years to determine how and when it was stripped away, the MAVEN mission is well-suited when it comes to measuring the potential habitability of other planets.
The study was shared on Dec. 13th, 2017, at the Fall Meeting of the American Geophysical Union in New Orleans, Louisiana. In a presentation titled “Spanning Disciplines to Search for Life Beyond Earth“, a team of NASA scientists and researchers from the University of California-Riverside and the University of Colorado-Boulder explained how insights from the MAVEN mission could be applied to the habitability of rocky planets orbiting other stars.
Artist’s rendering of a solar storm hitting Mars and stripping ions from the planet’s upper atmosphere. Credits: NASA/GSFC
Launched in November 18th, 2013, the MAVEN mission established orbit around Mars on September 22nd, 2014. The purpose of this mission has been to explore the Red Planet’s upper atmosphere, ionosphere and its interactions with the Sun and solar wind for the sake of determining how and when Mars’ atmosphere went from being thicker and warmer in the past (and thus able to support liquid water on the surface) to thin and tenuous today.
Since November of 2014, MAVEN has been measuring Mars’ atmospheric loss using its suite of scientific instruments. From the data it has obtained, scientists have surmised that the majority of the planet’s atmosphere was lost to space over time due to a combination of chemical and physical processes. And in the past three years, the Sun’s activity has increased and decreased, giving MAVEN the opportunity to observe how Mars’ atmospheric loss has risen and fallen accordingly.
Because of this, David Brain – a professor at the Laboratory for Atmospheric and Space Physics (LASP) at the CU Boulder is also a MAVEN co-investigator – and his colleagues began to think about how these insights could be applied to a hypothetical Mars-like planet orbiting around an red dwarf star. These planets include Proxima b (the closest exoplanet to our Solar System) and the seven planet system of TRAPPIST-1.
As Brain he explained in a recent NASA press release:
“The MAVEN mission tells us that Mars lost substantial amounts of its atmosphere over time, changing the planet’s habitability. We can use Mars, a planet that we know a lot about, as a laboratory for studying rocky planets outside our solar system, which we don’t know much about yet.”
At one time, Mars had a magnetic field similar to Earth, which prevented its atmosphere from being stripped away. Credit: NASA
To determine if this hypothetical planet could retain its atmosphere over time, the researchers performed some preliminary calculations that assumed that this planet would be positioned near the outer edge of the star’s habitable zone (as Mars is). Since red dwarf’s are dimmer than our Sun, the planet would have to orbit much closer to the star – even closer than Mercury does to our Sun – to be within this zone.
They also considered how a higher proportion of the light emanating from red dwarf stars is in the ultraviolet wavelength. Combined with a close orbit, this means that the hypothetical planet would be bombarded with about 5 times more UV radiation the real Mars gets. This would also mean that the processes responsible for atmospheric loss would be increased for this planet.
Based on data obtained by MAVEN, Brain and colleagues were able to estimate how this increase in radiation would affect Mars’ own atmospheric loss. Based on their calculations, they found that the planet’s atmosphere would lose 3 to 5 times as many charged particles through ion escape, while about 5 to 10 times more neutral particles would be lost through photochemical escape (where UV radiaion breaks apart molecules in the upper atmosphere).
Another form of atmospheric loss would also result, due to the fact that more UV radiation means that more charged particles would be created. This would result in a process called “sputtering”, where energetic particles are accelerated into the atmosphere and collide with other molecules, kicking some out into space and sending others crashing into neighboring particles.
To receive the same amount of starlight as Mars receives from our Sun, a planet orbiting an M-type red dwarf would have to be positioned much closer to its star than Mercury is to the Sun. Credit: NASA’s Goddard Space Flight Center
Lastly, they considered how the hypothetical planet might experience about the same amount of thermal escape (aka. Jeans escape) as the real Mars. This process occurs only for lighter molecules such as hydrogen, which Mars loses at the top of its atmosphere through thermal escape. On the “exo-Mars”, however, thermal escape would increase only if the increase in UV radiation were to push more hydrogen into the upper atmosphere.
In conclusion, the researchers determined that orbiting at the edge of the habitable zone of a quiet M-type star (instead of our Sun) could shorten the habitable period for a Mars-like planet by a factor of about 5 to 20. For a more active M-type star, the habitable period could be cut by as much as 1,000 times. In addition, solar storm activity around a red dwarf, which is thousands of times more intense than with our Sun, would also be very limiting.
However, the study is based on how an exo-Mars would fair around and M-type star, which kind of stacks the odds against habitability in advance. When different planets are considered, which possess mitigating factors Mars does not, things become a bit more promising. For instance, a planet that is more geologically active than Mars would be able to replenish its atmosphere at a greater rate.
Other factors include increase mass, which would allow for the planet to hold onto more of its atmosphere, and the presence of a magnetic field to shield it from stellar wind. As Bruce Jakosky, MAVEN’s principal investigator at the University of Colorado (who was not associated with this study), remarked:
“Habitability is one of the biggest topics in astronomy, and these estimates demonstrate one way to leverage what we know about Mars and the Sun to help determine the factors that control whether planets in other systems might be suitable for life.”
Multiple survey have revealed evidence of rocky planets orbiting a red dwarf stars, raising questions about their habitability. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).
In the coming years, astronomers and exoplanet researchers hope to learn more about the planets orbiting nearby red dwarf stars. These efforts are expected to be helped immensely thanks to the deployment of the James Webb Space Telescope, which will be able to conduct more detailed surveys of these star systems using its advanced infrared imaging capabilities.
These studies will allow scientists to place more accurate constraints on exoplanets that orbit red dwarf stars, which will allow for better estimates about their size, mass, and compositions – all of which are crucial to determining potential habitability.
Other panelists that took part in the presentations included Giada Arney and Katherine Garcia-Sage of NASA Goddard Space Flight Center and Stephen Kane of the University of California-Riverside. You can access the press conference materials by going to NASA Goddard Media Studios.
